1. Field of the Invention
This invention relates to the calibration of gamma spectrometer systems and, more particularly, to a method of producing liquid equivalent solid gamma ray calibration standards.
2. Discussion of the Prior Art
Gamma spectrometer systems using either NaI(Tl) or Ge(Li) detectors are used to perform qualitative and quantitative assays of liquid samples for radioactive material. Before the assays can be performed, the spectrometer system must be calibrated. This means that the relationship between gamma ray energy and analyzer channel number as well as the detector efficiency as a function of gamma ray energy must be known. These calibrations are best accomplished using standard gamma ray sources which emit gamma rays of known energy and have an accurately known gamma ray emission rate. With modern high resolution Ge(Li) gamma ray detectors, it is most economical to calibrate using mixed isotope sources which emit numerous gamma rays of different energies. The energy vs. channel number calibration is very easily performed by fitting a polynomial function to the energy vs. channel number data from a mixed source. In most systems, a linear relationship is sufficiently accurate but a quadratic or a cubic equation can be fit to the data for the most accurate calibration.
Unfortunately, the efficiency calibration of a gamma spectrometer system is more complicated than the energy calibration. The detector efficiency varies with the energy of the gamma ray and with the distance of the source to the detector. Variations of the detector efficiency with source distance produce variations in sample counting efficiency with container size and shape. Standards containing known quantities of radioactive material in each size and shape container must be measured in each counting position before quantitative analyses can be performed. Calibration standards must also be approximately the same density as the samples being analyzed to avoid the problem of correcting for differences in gamma ray scattering and absorption.
Liquid solutions of radioactive material commonly used as calibration standards suffer from several deficiencies. Liquid sources frequently leak or are spilled on radiation detectors, laboratory areas, or laboratory personnel. These types of accidents with high level radioactive material dissolved in chemically corrosive solutions can be very serious. More difficult to detect, low level-long term leakage from radioactive standard solutions has frequently contaminated radiation detection equipment to the point where it is useless in the measurement of low level radioactivity. In addition to these difficulties in the use and storage of liquid radioactive standards, disposal of expired liquid standards is also a problem. Current radioactive waste disposal regulations prevent most laboratories from directly disposing of the quantities of liquid radioactive material used as standards without large dilution tanks. Liquid radioactive waste generally must be solidified in some manner before disposal.
Aside from the question of safety, the preparation and storage of liquid radioactive standards is complicated by the solution chemistry of the elements involved. Mixtures containing widely different chemical species are particularly troublesome. Selective plate-out, precipitation, or volatilization can destroy the homogeneity of a liquid standard and render it useless for calibration purposes. The seven element mixture consisting of Cd, Co, Ce, Hg, Sn, Cs and Y commonly used in gamma ray calibration standards is plagued by problems of this kind. If the solution is weaker than 4 N in HCl, the Sn-113 will precipitate. Maintaining this high acidity for a long period of time in relatively porous plastic containers has been difficult. Also, certain types of plastic have been found to selectively remove tin from these standard solutions. Plate-out frequently occurs on glass surfaces where Y, Ce, and Cs are ion exchanged onto glass container surface, thus destroying the standard. Deposits of dirt or chemical residues from laboratory air can cause selective precipitation of some of the elements in a mixed standard. Consideration of these problems with liquid radioactive standards has led to the development of liquid equivalent solid radioactive sources with the radioactive material uniformly dispersed in a low density solid whose gamma ray attentuation properties closely match those of water.